New advancements in projection systems utilize an optical semiconductor known as a digital micromirror device. A digital micromirror device chip may be the world's most sophisticated light switch. It contains an array of from about 750,000 to about 1.3 million pivotally-mounted microscopic mirrors. Each mirror may measure less than ⅕ of the width of a human hair and corresponds to one pixel in a projected image. The digital micromirror device chip can be combined with a digital video or graphic signal, a light source, and a projector lens so that the micromirrors reflect an all-digital image onto a screen or other surface.
Although there are a variety of digital micromirror device configurations, typically micromirrors are mounted on tiny hinges that enable each mirror to be tilted either toward the light source (on) in a projector system to reflect the light; or away from the light source (off) to create a darker pixel on the projection surface. A bitstream-to-image code entering the semiconductor directs each mirror to switch on or off several times per second. When the mirror is switched on more frequently than off, the mirror reflects a light gray pixel. When the mirror is switched off more frequently than on, the mirror reflects a darker gray pixel. Some projection systems can deflect pixels enough to generate 1,024 shades of gray to convert the video or graphic signal entering the digital micromirror device into a highly-detailed grayscale image. In some systems, light generated by a lamp passes through a color wheel as it travels to the surface of the digital micromirror device panel. The color wheel filters the light into red, green and blue. A single-chip digital micromirror vice projector system can create at least 16.7 million colors. When three digital micromirror device chips are utilized, more than 35 trillion colors can be produced. The “on” and “off” states of each micromirror are coordinated with the three basic building blocks of color (red, green and blue) to produce a wide variety of colors.
A variety of digital micromirror devices (DMD) are known. FIG. 1 illustrates one embodiment of a prior art DMD that may be used in the present invention with the substitution of a unique mirror structure according to the present invention. As shown in FIG. 1, a DMD 10 may include a semiconductor device 12 such as a CMOS memory device that includes circuitry 13 that is used to activate an electrode(s) in response to a video or graphic signal. A first layer 14 is formed over the semiconductor device 12 and may include a yoke address electrode 16, vias 18 formed therein down to the circuitry 13 on the semiconductor device 12, and a bias-reset bus 20. A second layer 22 is formed over the first layer 14 and may include a yoke 24, a torsion hinge 26 and mirror address electrodes 28. A micromirror 32 is formed over the second layer 22 and positioned so that the micromirror 32 may be deflected diagonally when one of the electrodes 28 is activated by the semiconductor device 12. The micromirror 32 includes a reflective layer typically including aluminum. The DMD 10 shown in FIG. 1, while being an excellent engineering accomplishment, is very complex, costly to manufacture and has a low manufacturing yield. Further, the micromirror 32 may include defects, as will be described hereinafter with respect to a second configuration of a DMD.
FIG. 2 illustrates a first subassembly 40 for a second type of DMD. The subassembly 40 may include a transparent layer 42 which may be any transparent material including, but not limited to, glass. A hinge 44 is formed on the transparent layer 42 and a micromirror 32 is secured thereto for pivotal movement with respect to the hinge 44 and the transparent layer 42.
FIG. 3 illustrates the first subassembly 40 including a plurality of micromirrors 32, each connected by a hinge 44 to the transparent layer 42. All of the components and subassemblies of the various DMD devices can be made by semiconductor or MEM micro processing techniques known to those skilled in the art.
FIG. 4 illustrates a second subassembly 46 of the second type of DMD and may include a semiconductor device 12 such as, but not limited to, a CMOS memory device. A plurality of electrodes 48, one for each micromirror 32, are formed over the semiconductor device 12 for communication with the circuitry (not shown) contained therein so that the electrode 48 may be selectively activated in response to a video or graphic signal.
FIG. 5 illustrates a DMD structure 10 that may be utilized by the present invention with the substitution of a unique micromirror structure according to the present invention. The DMD of FIG. 5 includes the first subassembly 40 flipped over and overlying the second subassembly 46 so the micromirrors 32 of the first subassembly 40 face and are closest to the electrodes 48 of the second subassembly 46. Spacers 50 are provided so that the micromirrors 32 are spaced a distance from the electrodes 48 and so that each micromirror 32 is free to be deflected or pivotally-moved by the activation of an associated electrode 48. As illustrated in FIG. 5, when light is directed onto the micromirrors 32, an electrode 48 associated with each micromirror 32 may be activated to cause the micromirror 32 to pivotally move about the hinge 44. As a result, the light will be reflected or not depending on whether or not the electrode 48 associated with the micromirror 32 has been activated. As described above, depending on how fast and how often a particular micromirror 32 is deflected by the corresponding electrode 48, the image projected by the micromirror 32 (pixel) will appear light or dark on the projection screen (not shown) or other surface.
Conventional micromirrors often include hillocks (raised features or bumps) 54 or voids 52 in the aluminum layer, as shown in FIGS. 6 and 7. Typically, the micromirror 32 includes a sputtered-on-aluminum coating which may often include hillocks 54 or voids 52. The hillocks 54 or voids 52 can cause artifacts or distortions in the projected image.
FIG. 8 illustrates a typical multi-layered structure of a conventional micromirror 32. The micromirror 32 includes a substrate 60, which is typically glass; a first protective layer 62, which is typically PEOX (plasma-enhanced oxide), deposited on the substrate 60; a reflective layer 64, which is typically AlSiCu, deposited on the first protective layer 62; a treatment layer 66, typically titanium (Ti), deposited on the reflective layer 64; and a second protective layer 68, typically PEOX, deposited on the treatment layer 66.
One of the problems associated with the use of AlSiCu as the reflective layer 64 is that metal pits tend to form in the reflective layer 64. Furthermore, the silicon tends to precipitate in the reflective layer 64, causing unstable contrast ratios of light reflected from the micromirror 32. Use of pure aluminum for the reflective layer 64 imparts severe metal roughness to the surface of the reflective layer 64, thus distorting the light reflected from the micromirror 32. Moreover, the use of PEOX for the second protective layer 68 provides an unstable mirror spacer etching stop point, leading to compromised CID uniformity.
It has been found that the use of pure aluminum (Al) as the reflective layer 64 substantially reduces or eliminates the formation of pits in the reflective layer 64. Furthermore, it has been found that deposition of TiN at room temperature as the second protective layer 68 substantially reduces or eliminates surface roughness in the reflective layer 64. Moreover, the TiN second protective layer 68 functions as an effective mirror spacer etching stop layer.
Accordingly, an object of the present invention is to provide a novel micromirror having enhanced reflective characteristics.
Another object of the present invention is to provide a novel micromirror characterized by reduced voids or pits.
Still another object of the present invention is to provide a novel micromirror characterized by reduced surface roughness.
Yet another object of the present invention is to provide a novel micromirror having a reflective layer which may be pure aluminum (Al) and a protective layer which may be titanium nitride (TiN).
A still further object of the present invention is to provide a novel micromirror having a reflective layer which is substantially devoid of precipitates.
Another object of the present invention is to provide a micromirror having a protective layer which functions as an effective mirror spacer etching stop layer.